Sand casting, also known as sand molded casting, is a process for casting parts, normally metal parts, characterized by using sand as the mold material. A suitable bonding agent is mixed with the sand to develop coherency for molding and strength and stiffness of the cured mold.
For manufacturing metal objects, the basic steps of the sand casting process are quite simple. A pattern is made for the object to be produced, typically using wood, metal, or a plastic. The pattern is placed in a suitable sand mixture, contained and cured in a casting box, to create a sand mold. The pattern is removed, to form the mold cavity, and the mold cavity is filled with molten metal. After the metal cools, the sand mold is broken away leaving the desired casting.
To produce internal holes and passages within the casting, “cores” are used. A core is formed independently of the sand mold, usually also from sand, then positioned in the mold cavity, with some means for supporting the core in position. The positioning means may be one or more recesses in the pattern called “core prints” or small supporting pieces between the core and cavity surface called “chaplets”. Then, the molten metal is introduced as described above.
Although sand cores are useful, the cross section size of the internal passages made using sand cores is limited. This is because as sand core cross section dimensions are reduced, the core's ability to resist premature breakdown in the presence of molten metal is also reduced. Thus, there are limiting dimensions below which a sand core will disintegrate during casting by effects that include thermal shock, evaporation of binder and penetration of the sand core.
Internal combustion engines contain numerous flow passages for delivery of fluids (such as fuel, lubricant, coolant and air) to various locations throughout the engine. It is desirable that as many of these passages as possible be contained within the cast material of components such as the cylinder block and cylinder head, to avoid external plumbing and additional parts count.
However, many of these engine passages are too small to cast using conventional sand core casting methods, and therefore they must be machined separately after the components are cast. This normally requires a sequence of machined features, typically drillings, which intersect to create flow networks. Because machining requires straight “line-of-sight” access to locate the features, the flexibility of their placement, orientation and shape is very limited. Additionally, in some engines the passages are long and consequently difficult to machine. Also, many of these drillings must be plugged to seal one end, which requires further machining and creates potential fluid leak paths.